Civ5/UnCiv is known for being not focused on city planning like civ6 is(500 map tacks on turn 2). But this does not mean there is no planning you need to do, and if you plan your cities poorly you will lose out on yields.

The first thing that is important to talk about is how far apart you want your cities. You can build your cities between 3-6 tiles apart, anymore and you can't work the tiles and you can build a city in
between. It is also important to note, if there are mountain, coast or ocean tiles in between 2 landmasses you can build a city 2 spaces apart(but not 1). Yes, El Dorado counts as a flat tile even though it's impassible.

When settling, you always want to prioritize settling a coastal river, even if it means you could possibly lose out on a city placement. Rivers are hugely important but often forgotten feature of the game. They will make all of your river tiles so much more profitable once you build a water mill and a hydro plant, having the potential to earn up to 20 extra production from your tiles.

Building cities the minimum space apart is not a terrible idea, especially if you're strapped for space on your map and there won't be too much settling to do. Because all of your buildings are contained inside of the city and don't take up any tiles like they do in civ6, building as many cities as possible is a viable strat and will reward you well. This is known as building wide.

If you're playing in larger maps and you can pick and choose the best places to settle, building cities more than the minimum apart is also an excellent idea. Especially since you probably won't finish settling until the late game, this strategy is known as building tall. Founding your cities 6 tiles apart will allow each city to work the maximum amount of possible tiles it can. Because you founded less cities than someone who built wide, it will be much easier to build what you want in the cities you have and build national wonders, which you can't usually get until the late game if you settle as much as you possibly can. This also creates synergy really well with the tradition policy branch along with civs such as Ethiopia and India.

It is very situational which strategy is best, it really comes down to how much landmass you have to settle(and how many happiness hits you can take). Keep in mind certain wonders are more effective depending on your settling style(I'll get to that later.)

And now to talk about wonders. If you're playing an easier difficulty, you'll be able to build most if not all of the wonders. If you're playing a harder difficulty, you should pick and choose the best ones. Most wonder placement don't have too large an impact on your games, ideally you want to build all of your yield-giving wonders and national wonders that do the same in 1 city so that you can get the most out of buildings like the stock exchange that increase your stats on a percentage basis, but this will give you marginal results and isn't all that important. Building a wonder before your opponent can often be more important that where it is built.

A few exceptions to this rule are: Petra, Alhambra, the great mosque of Djenne and the Brandenburg Gate. While placement of wonders such as ToA and Sydney Opera house can make a huge difference in their percentage bonuses depending on where you build them, but that won't give you as inherent a bonus as these wonders.

Petra is hugely important to build in a city with a lot of desert that you build to be tall(The AI with 1 Desert tile stole this from me once) You want to avoid settling cities in the immediate vicinity of the city that has this, as the production, food and gold bonuses apply for owning a tile and not working it. If you build this wonder, wait until you annex all the tiles it can reasonably expect to annex and then settle around it Petra will make you more yields than you could possibly imagine on any other kind of terrain. Keep in mind, this will apply to all kinds of desert tiles.

It is crucial for Alhambra and the Brandenburg gate to be built in the same city along the coast. If you also build a military academy and a heroic epic in that city, you can get the most well trained units you can possibly get without actually fighting anybody(you can pick up an additional 15 xp from autocracy if you really feel like it). As it gets later and later into the game, the percentage damage bonuses from promotions become more and more effective as units get stronger. It truly takes forever to actually earn promotions past your second, and you can't roll that out across your army like you can if you do this. The downside though is if you want that level of training to be standard in your forces you will only be able to build military units in 1 city though.

The Great mosque is less important in it's placement more than it is helpful. If you built this wonder, you should not be training great prophets or missionaries in any other city. That extra charge will be priceless in a battle of religious control. Ideally you want to build this in a city close to another civ to shorten your supply lines to the fronts of religious combat.

Keep in mind this is only my personal opinion and I am certainly no expert at this game. If there's anything I forgot, be sure to let me know. I have roughly 600 hours of experience in UnCiv alone, and I'm always looking to improve my knowledge of the game.